Future wireless systems require efficient utilization of the radio frequency spectrum in order to increase the data rate achievable within a given transmission bandwidth. This can be accomplished by employing multiple transmit and receive antennas combined with signal processing. A number of recently developed techniques and emerging standards are based on employing multiple antennas at a base station to improve the reliability of data communication over wireless media without compromising the effective data rate of the wireless systems. So called space-time block-codes (STBCs) are used to this end.
Specifically, recent advances in wireless communications have demonstrated that by jointly encoding symbols over time and transmit antennas at a base station one can obtain reliability (diversity) benefits as well as increases in the effective data rate from the base station to each wireless user. These multiplexing (throughput) gain and diversity benefits depend on the space-time coding techniques employed at the base station. The multiplexing gains and diversity benefits are also inherently dependent on the number of transmit and receive antennas in the system being deployed, in the sense that they are fundamentally limited by the multiplexing-diversity trade-offs curves that are dictated by the number of transmit and the number of receive antennas in the system.
For high data rates and wideband transmission, the use of OFDM makes the equalizer unnecessary. With multilevel modems, coded modulation systems can easily be designed by use of an outer binary code, such as a convolutional code, and an interleaver in a so called bit-interleaved coded modulation (BICM) system.
There are a number of existing receiver structures for the transmission systems that combine the use of MIMIO, OFDM, and BICM, that is, for coded MIMO/OFDM/BICM/ID systems. These receivers include an inner-outer decoder structure, whereby the outer decoder is optimally selected. The receivers include iterative decoding (ID) receivers, ID receivers with a MaxLogMAP-based inner decoder, receivers using QRD/M-Algorithm based inner decoder, receivers having MMSE-based inner decoders, and receivers that employ a tree search based on conventional SOMA.
Iterative decoding (ID) receivers with a MAP-based inner decoder have the optimum bit-error-rate performance among all inner/outer decoder structures. However, the MAP-based inner decoder becomes computationally intractable as N (number of transmit antennas/number of QAM symbols that need to be jointly resolved) and b (number of bits represented by each QAM symbol) increase.
ID receivers with a MaxLogMAP-based inner decoder are less complex than the MAP-based system and is asymptotically (high SNR) optimal in that it has near optimum bit-error-rate performance at high SNR. However, the maxlogMAP-based inner decoder also becomes computationally intractable as N and b increase.
Receivers using QRD/M-Algorithm based inner decoder use a variant of the M-algorithm to produce hard bit estimates along with reliability (soft-output) information. As a result, they can yield drastic reductions in complexity by proper choice of the M parameter, at a cost in bit-error-rate performance. These methods directly employ the conventional M-algorithm, to generate hard-output estimates, and then employ the resulting M candidates to obtain soft information. However, to generate soft information for any bit location, both values of the bit must be available in the pool of the remaining M candidates. As a result, these methods resort to heuristic (and inferior) softify-ing techniques to generate soft output for each bit location for which only one bit value is available in the pool of remaining M candidates. In addition, these receiver structures do not exploit iterative decoding.
Receivers with MMSE-based inner decoders have much lower complexity but suffer in bit-error-rate performance, especially, at higher outer-code rates.
Receivers that employ a tree search based on conventional soft-output M-algorithm, SOMA, do not suffer from having to resort to heuristics for computing reliability information. Instead, in SOMA receivers reliability values are calculated recursively in the forward direction as the M-algorithm is implemented on the tree. These SOMA receivers can also be used for iterative decoding. However, since conventional SOMA receivers recursively compute reliability information in the forward direction, they will sometimes yield reliability values which are not calculated relative to the globally best sequence that is provided by the M-algorithm.
The above set of receiver structures is not exhaustive, but rather representative. There exist many other inner decoder structures in the literature, including spherical decoders, soft-output Viterbi-algorithm (SOVA) based inner-decoders, etc.